The present invention relates to drive axle systems for vehicles such as 6 .times. 6 vehicles, and more particularly, to an improved system for providing torque transmission to the front drive-steer axle.
A 6 .times. 6 vehicle is one having six wheels, all of which are driven, and abbreviatons such as 6 .times. 6 will be used hereinafter, the first numeral denoting the total number of vehicle wheels and the second numeral denoting the number of driven wheels. Therefore, although the present invention may be utilized with vehicles having a larger number of wheels, such as an 8 .times. 8 vehicle, as long as the front axle is driven, the invention is especially adapted for use with a 6 .times. 6 vehicle, and will be described in connec tion therewith.
Under normal driving conditions, a 6 .times. 6 vehicle operates as a 6 .times. 4 vehicle, i.e., the vehicle transmission delivers torque to the tandem axle assembly which in turn delivers the torque to the two rear axles, with the front drive-steer axle not being driven. It will become apparent, however, that the invention is equally adapted to a vehicle which operates, even under normal conditions, as a 6 .times. 6.
The essence of a tandem axle assembly is its ability to drive more than one drive axle, generally two or three rear axles. This is typically accomplished by means of a power (or torque) divider, usually a differential, in which one of the differential side gears transmits, under ideal operating conditions as hereinafter defined, a certain percentage of the input torque to the first drive axle while the other differential side gear transmits the remaining torque by means of an output shaft to a subsequent axle assembly including a second drive axle. A tandem assembly of the type well known in the art is illustrated in U.S. Pat. No. 3,000,456, assigned to the assignee of the present invention and which is incorporated herein by reference. The term "ideal operating conditions" as used herein shall refer to a situation wherein all driven wheels are of substantially equal radius and are smoothly rolling on the same substantially flat surface with no slippage occurring.
6 .times. 6 VEHICLE DRIVE SYSTEMS PRESENTLY KNOWN IN THE ART UTILIZE A TRANSFER CASE MOUNTED EITHER ON THE REAR SIDE OF THE VEHICLE TRANSMISSION, DRIVEN DIRECTLY BY THE TRANSMISSION OUTPUT SHAFT, OR MOUNTED TO THE VEHICLE CHASSIS AND DRIVEN BY A DRIVE LINE ATTACHED TO THE TRANSMISSION OUTPUT SHAFT AND THE TRANSFER CASE INPUT SHAFT. Examples of conventional 6 .times. 6 drive systems and the transfer cases utilized therein are shown in U.S. Pat. Nos. 2,770,150; 3,191,708; and 3,495,477. The use of such a transfer case involves additional gearing and substantially increases the weight and cost of such drive systems. Also, it is required that the transfer case be quite large to enable the output shaft from the transfer case to the front drive-steer axle to clear the transmission bell housing.
Accordingly, it is an object of the present invention to provide a vehicle drive system which is capable of transmitting torque to a front drive-steer axle, and which eliminates the necessity for a transfer case and the associated space requirements and additional weight and cost.
It is a more specific object of the present invention to provide such a drive system in a vehicle having a tandem axle assembly in which the gearing in the tandem axle assembly is directly utilized to provide the power take-off to drive the front drive-steer axle.
A further disadvantage of prior art 6 .times. 6 drive systems relates to the fact that the transfer case, when driving the front axle, must, under ideal operating conditions, generally transmit 50% of the torque to the tandem axle assembly and a 50% of the torque to the front axle. Such a distribution of the torque is disproportionate to the axle loadings in which, typically, 30% of the weight is on the front axle and 70% of the weight is on the rear axles. This 50- 50% torque distribution remains generally constant until one of the axles loses traction, which would normally be the front drive-steer axle, because the proportion of the available torque transmitted to the front axle is greater than the proportion of the total weight on that axle. This fact places an undesirably low limit on the total torque which may be transmitted to the ground up to the point of loss of traction.
Accordingly, it is an object of the present invention to provide a 6 .times. 6 vehicle drive system in which the torque distribution is more nearly in conformity with the load distribution than has been possible with prior art drive systems.
It is a related object of the present invention to provide a vehicle drive system which achieves the above-stated object and increases the total torque output to the ground without loss of traction.